Lee waves over the English Lake district

This paper presents the results of a field experiment which was conducted for three weeks during November 1991. The aim was to investigate the properties of internal gravity waves in the troposphere and lower stratosphere which were generated by the mountains of the Lake District in north-west England, UK. The main technique used was that of multiple sounding using radiosondes. Throughout the experiment measurements obtained from groups of five radiosondes launched nearly simultaneously indicate that the waves are quasi-steady and trapped within the troposphere. This is illustrated by a single detailed case study for 26 November in which the radiosonde ascent-rate fluctuations are as large as 6.5 m s(-1) peak to peak. These measurements are supported by aircraft observations. Simple techniques for analysing the structure of the wave-field are presented and these include the calculation of a phase-line tilt and phase speed and comparisons with simple analytical and numerical solutions for trapped gravity waves. A three-dimensional linear numerical model is used to simulate the waves observed on 26 November and three other cases also. The model predictions are shown to compare well with the observations. This is particularly true in the mid-troposphere, where the model reproduces both the positions and amplitudes of the peaks and troughs in the vertical-velocity field. Spectral analysis of the local orography and examination of typical variations of the wind and buoyancy frequency with height show that gravity waves generated by the orography in the Lake District will frequently be trapped in the troposphere.